1. Field of the Invention
The present invention relates to an image forming apparatus that controls an image processing unit by using information obtained by a color discrimination method of an electrophotographic or inkjet color image forming apparatus, such as a color printer and a color copy machine.
2. Description of the Related Art
In recent years, higher image quality of an output image has been demanded for an electrophotographic or inkjet color image forming apparatus such as a color printer and a color copy machine. Particularly, in the case of the electrophotographic color image forming apparatus, a constant gradation-density characteristic needs to be maintained all the time, because the color balance may be disrupted due to a variation in the density of an image caused by a change of the environment or a variation in each part of the apparatus due to a long time usage. To this end, a density sensor or a chromaticity sensor (hereinafter referred to as “color sensor”) for detecting a density of a monochrome toner image or a color of a full color image on a transfer material after transferring and fixing toner images is installed in the color image forming apparatus. A color toner patch (hereinafter referred to as “patch”) for controlling the density or the chromaticity is formed on the transfer material, and the density or the chromaticity detected by the color sensor is fed back to a process condition, such as an exposure amount, a process setting, or a look-up table (LUT). With this configuration, the density or the chromaticity of the final output image formed on the transfer material is controlled. For example, a gamma characteristic control is performed based on the measured density, or a correction of a color matching table or a color separation table is performed based on the measured chromaticity. The same goes for an inkjet printer.
In order to detect the density or the chromaticity of the patch with the color sensor, a white reference plate or the like for correcting a sensor output is required for the following reasons. The first reason is that a fluctuation of spectroscopic characteristics of a light emitting element and a light receiving element constituting the sensor needs to be corrected. The second reason is that, even when the same patch is used for the detection, different outputs may be produced due to a temporal change of a light emitting unit or a light receiving unit constituting the sensor or a change of the ambient temperature. The third reason is that, because a large number of transfer materials pass near the sensor at the time of normal printing, paper dust, toner, or ink may be scattered and deposited or adhered to a surface of the sensor, causing a degradation of the sensor output. In addition, in order to detect the absolute density or the absolute chromaticity of the patch by using the color sensor, there is required a white reference plate or the like for correcting the sensor output, which has a known absolute value of the density or the chromaticity.
For example, in Japanese Patent Application Laid-Open No. 2003-149903, a process of measuring the density or the chromaticity illustrated in FIG. 9 is executed by using a filter-type color sensor. Firstly, in Steps S1001 and S1002, a process of adjusting the color sensor is executed based on an absolute reference such as the white reference plate. Specifically, in Step S1001, an RGB output measured value Dref(i) (i=r, g, and b, the same applies hereinafter) of the absolute reference is detected by the color sensor. In Step S1002, a correction coefficient Oref(i)/Dref(i) is obtained from the RGB output measured value Dref(i) of the detected absolute reference and an RGB output theoretical value Oref(i) stored in advance. In Steps S1003 and S1004, a process of measuring the absolute density or the absolute chromaticity by the adjusted color sensor is executed. Specifically, in Step S1003, an RGB output measured value D(i) of a patch is detected by the color sensor. In Step S1004, the RGB output measured value D(i) of the detected patch is converted into an RGB output value O(i) corrected by using the absolute reference. The RGB output value O(i) is obtained by uniformly multiplying the RGB output measured value D(i) of the detected patch by the correction coefficient Oref(i)/Dref(i) as represented by Equation (1).O(i)=D(i)×Oref(i)/Dref(i)(i=r,g, and b)  Equation (1)
Similarly, in a measurement of the density and the chromaticity by a spectrophotometric color sensor, a process of adjusting the color sensor by using the absolute reference and a process of measuring spectroscopic reflectivity by using the adjusted color sensor are executed. It is assumed that a spectroscopic output measured value of the absolute reference is represented by Dref(m), a spectroscopic output theoretical value is represented by Oref(m), a spectroscopic output measured value of a patch is represented by D(m), and a spectroscopic output value of the patch corrected by using the absolute reference is represented by O(m), provided that, m represents a wavelength in the visible light wavelength band. For example, m indicates an arbitrary wavelength in a wavelength range from 380 nm to 730 nm. As represented by Equation (2), the obtained spectroscopic output measured value D(m) of the patch is converted into the spectroscopic output value O(m) corrected by using the absolute reference. The spectroscopic output value O(m) is obtained by uniformly multiplying the obtained spectroscopic output measured value D(m) of the patch by a correction coefficient Oref(m)/Dref(m).O(m)=D(m)×Oref(m)/Dref(m)(m=380,390, . . . , 730)  Equation (2)
In the following, the reference plate, which is the absolute reference, is described as white. However, the reference plate is not limited to white, but may be any color so long as the absolute value of the density or the chromaticity is known.
However, the white reference plate that is generally and commonly used as the absolute reference for correcting the output of the color sensor may be discolored compared to the initial white reference plate due to yellowing generated after a long time irradiation with light from the color sensor, for example. Further, in the same manner as the color sensor, the white reference plate may become unusable as the absolute reference due to the scattering of paper dust, toner, or ink. If the correction of the color sensor output is performed in a state in which the white reference plate is stained and discolored, the color sensor ends up with outputting a value that is different from the actual density or chromaticity of the patch. If a control of the density or chromaticity is performed by using the result, the color balance cannot be obtained, and as a result, a desired gradation-density characteristic cannot be obtained. Not only that, the gradation-density characteristic is likely to disrupt the color balance.